Black-and-white silver halide photography has relied traditionally upon developed silver to produce a viewable image. Although black-and-white photography serves a variety of imaging needs, medical radiography, described below, illustrates the varied and in some instances competing demands that are encountered in silver imaging.
In medical radiography comparatively large areas of the radiation-sensitive material are often required for a single exposure--i.e., large format exposures are common. Further, the silver which remains in the element for imaging may be unavailable for reclamation for many years. Therefore, it is highly desirable to make efficient use of the silver which the radiographic elements contain. One measure of the efficiency of silver use is covering power. Covering power is herein defined as 100 times the ratio of maximum density to developed silver, expressed in grams per square decimeter. High covering power is recognized to be an advantageous characteristic of not only radiographic elements, but other black-and-white photographic elements as well. Covering power and conditions which affect it are discussed by James, Theory of the Photographic Process, 4th Ed., Macmillan, 1977, pp. 404, 489, and 490, and by Farnell and Solman, "The Covering Power of Photographic Silver Deposits I. Chemical Development", The Journal of Photographic Science, Vol. 18, 1970, pp. 94-101.
One approach to achieving high covering power is to employ relatively fine silver halide grains, since it is well recognized that increasing grain size will reduce covering power. Unfortunately, in medical radiography even more important than achieving efficient use of silver is the need to minimize patient exposure to X-radiation. Since silver halide becomes more sensitive (increases in speed) as a direct function of grain size, it is not then surprising that radiographic elements commonly employ large grain sizes. Thus, although attaining high covering power is important, the comparatively coarse silver halide emulsions actually employed are not well suited to achieving high levels of covering power.
Other techniques are therefore employed to improve covering power. It is known that larger silver halide grain sizes, typically at least about 0.6 micron in average diameter and larger, are subject to reductions in covering power as a function of hardening. To achieve the highest covering power compatible with speed requirements (and therefore grain size requirements), it is common practice in the art to limit forehardening (i.e., hardening during manufacture) to just the degree necessary to permit the radiographic elements to be handled (although the risk of damage of such materials remains comparatively high).
Final hardening to the desired level is achieved by incorporating a hardener in the processing composition, usually the developer. Particularly effective hardeners for use in processing compositions are dialdehydes and bis-bisulfite derivatives thereof of the type disclosed in Allen and Burness U.S. Pat. No. 3,232,764. Unfortunately, the hardener must be kept separate from the developer composition prior to use. Further, the presence of such hardener places additional constraints on the choice of developer compositions.
In a typical medical radiographic application a radiographic film is employed having relatively coarse silver halide emulsions coated on both major surfaces. The emulsion layers are minimally forehardened to achieve maximum covering power. The element is more sensitive to light than to X-radiation and is therefore typically placed between a pair of fluorescent screens which, upon imagewise exposure to X-radiation, imagewise fluoresce to expose the radiographic element. Thereafter the radiographic element is processed in a developer containing a hardener. To provide rapid access to a viewable image, the radiographic element is processed at temperatures above ambient (typically about 25.degree. to 50.degree. C.) and in time periods of less than 1 minute. Development is usually complete in about 20 seconds. A typical process of the type described above is illustrated by Barnes et al U.S. Pat. No. 3,545,971.
A great variety of regular and irregular grain shapes have been observed in silver halide photographic emulsions intended for black-and-white imaging applications generally and radiographic imaging applications specifically. Regular grains are often cubic or octahedral. Grain edges can exhibit rounding due to ripening effects, and in the presence of strong ripening agents, such as ammonia, the grains may even be spherical or near spherical thick platelets, as described, for example by Land U.S. Pat. Nos. 3,894,871 and Zelikman and Levi Making and Coating Photographic Emulsions, Focal Press, 1964, page 223. Rods and tabular grains in varied portions have been frequently observed mixed in among other grain shapes, particularly where the pAg (the negative logarithm of silver ion concentration) of the emulsions has varied during precipitation, as occurs, for example in single-jet precipitations.
Tabular silver bromide grains have been extensively studied, often in macro-sizes having no photographic utility. Tabular grains are herein defined as those having two substantially parallel crystal faces, each of which is substantially larger than any other single crystal face of the grain. The aspect ratio--that is, the ratio of diameter to thickness--of tabulator grains is substantially greater than 1:1. High aspect ratio tabular grain silver bromide emulsions were reported by de Cugnac and Chateau, "Evolution of the Morphology of Silver Bromide Crystals During Physical Ripening", Science et Industries Photographiques, Vol. 33, No. 2 (1962), pp. 121-125.
From 1937 until the 1950's the Eastman Kodak Company sold a Duplitized.RTM. fully forehardened radiographic film product under the name No-Screen X-Ray Code 5133. The product contained as coatings on opposite major faces of a film support sulfur sensitized silver bromide emulsions. Since the emulsions were intended to be exposed by X-radiation, they were not spectrally sensitized. The tabular grains had an average aspect ratio in the range of from about 5 to 7:1. The tabular grains accounted for greater than 50% of the projected area while nontabular grains accounted for greater than 25% of the projected area. The emulsion having the thinnest average grain thickness, chosen from several remakes, had an average tabular grain diameter of 2.5 microns, an average tabular grain thickness of 0.36 micron, and an average aspect ratio of 7:1. In other remakes the emulsions contained thicker, smaller diameter tabular grains which were of lower average aspect ratio.
Although tabular grain silver bromoiodide emulsions are known in the art, none exhibit a high average aspect ratio. A discussion of tabular silver bromoiodide grains appears in Duffin, Photographic Emulsion Chemistry, Focal Press, 1966, pp. 66-72, and Trivelli and Smith, "The Effect of Silver Iodide Upon the Structure of Bromo-Iodide Precipitation Series", The Photographic Journal, Vol. LXXX, July 1940, pp. 285-288. Trivelli and Smith observed a pronounced reduction in both grain size and aspect ratio with the introduction of iodide. Gutoff, "Nucleation and Growth Rates During the Precipitation of Silver Halide Photographic Emulsions", Photographic Sciences and Engineering, Vol. 14, No. 4, July-August 1970, pp. 248-257, reports preparing silver bromide and silver bromoiodide emulsions of the type prepared by single-jet precipitations using a continuous precipitation apparatus.
Bogg, Lewis, and Maternaghan have recently published procedures for preparing emulsions in which a major proportion of the silver halide is present in the form of tabular grains. Bogg U.S. Pat. No. 4,063,951 teaches forming silver halide crystals of tabular habit bounded by {100} cubic faces and having an aspect ratio (based on edge length) of from 1.5 to 7:1. The tabular grains exhibit square and rectangular major surfaces characteristic of {100} crystal faces. In the example reported the average edge length of the grains was 0.93 micron and the average aspect ratio 2:1. Thus the average grain thickness was 0.46 micron, indicating thick tabular grains were produced. Lewis U.S. Pat. No. 4,067,739 teaches the preparation of silver halide emulsions wherein most of the crystals are of the twinned octahedral type by forming seed crystals, causing the seed crystals to increase in size by Ostwald ripening in the presence of a silver halide solvent, and completing grain growth without renucleation or Ostwald ripening while controlling pBr (the negative logarithm of bromide ion concentration). Maternaghan U.S. Pat. Nos. 4,150,994, 4,184,877, and 4,184,878, U.K. Pat. No. 1,570,581, and German OLS publications 2,905,655 and 2,921,077 teach the formation of silver halide grains of flat twinned octahedral configuration by employing seed crystals which are at least 90 mole percent iodide. (Except as otherwise indicated, all references to halide percentages are based on silver present in the corresponding emulsion, grain, or grain region being discussed.) Lewis and Maternaghan report increased covering power. Maternaghan states that the emulsions are useful in camera films, both black-and-white and color. It appears from repeating examples and viewing the photomicrographs published that average tabular grain thicknesses were greater than 0.40 micron. Japanese patent Kokai 142,329, published Nov. 6, 1980, appears to be essentially cumulative with Maternaghan, but is not restricted to the use of silver iodide seed grains. Thus, the patents discussed above in this paragraph are viewed as teaching the preparation of silver halide emulsions containing relatively thick tabular grains of intermediate average aspect ratios.
Wilgus and Haefner U.S. Ser. No. 429,420, filed concurrently herewith and commonly assigned, titled HIGH ASPECT RATIO SILVER BROMOIODIDE EMULSIONS AND PROCESSES FOR THEIR PREPARATION, which is a continuation-in-part of U.S. Ser. No. 320,905, filed Nov. 12, 1981, now abandoned, more fully discussed below, discloses high aspect ratio silver bromoiodide emulsions and a process for their preparation.
Kofron et al U.S. Ser. No. 429,407, filed concurrently herewith and commonly assigned, titled SENSITIZED HIGH ASPECT RATIO SILVER HALIDE EMULSIONS AND PHOTOGRAPHIC ELEMENTS, which is a continuation-in-part of U.S. Ser. No. 320,904, filed Nov. 12, 1981, now abandoned, more fully discussed below, discloses chemically and spectrally sensitized high aspect ratio tabular grain silver halide emulsions and photographic elements incorporating these emulsions.
Daubendiek and Strong U.S. Ser. No. 429,587, filed concurrently herewith and commonly assigned, titled AN IMPROVED PROCESS FOR THE PREPARATION OF HIGH ASPECT RATIO SILVER BROMOIODIDE EMULSIONS, which is a continuation-in-part of U.S. Ser. No. 320,906, filed Nov. 12, 1981, now abandoned, more fully discussed below, discloses an improvement on the processes of Maternaghan whereby high aspect ratio tabular grain silver bromoiodide emulsions can be prepared.
Abbott and Jones U.S. Ser. No. 430,222, filed concurrently herewith and commonly assigned, titled RADIOGRAPHIC ELEMENTS EXHIBITING REDUCED CROSSOVER, which is a continuation-in-part of U.S. Ser. No. 320,907, filed Nov. 12, 1981, more fully discussed below, discloses the use of high aspect ratio tabular grain silver halide emulsions in radiographic elements coated on both major surfaces of a radiation transmitting support to control crossover.
Abbott and Jones U.S. Ser. No. 431,910, filed concurrently herewith and commonly assigned, titled RADIOGRAPHIC ELEMENTS EXHIBITING REDUCED CROSSOVER, more fully discussed below, discloses the use of thin, intermediate aspect ratio tabular grain silver halide emulsions in radiographic elements coated on both major suirfaces of a radiation transmitting support to control crossover.
Wey U.S. Ser. No. 429,403, filed concurrently herewith and commonly assigned, titled IMPROVED DOUBLE-JET PRECIPITATION PROCESS AND PRODUCTS THEREOF, which is a continuation-in-part of U.S. Ser. No. 320,908, filed Nov. 12, 1981, now abandoned, more fully discussed below, discloses a process of preparing tabular silver chloride grains which are substantially internally free of both silver bromide and silver iodide. The emulsions have an average aspect ratio of greater than 8:1.
Solberg, Piggin, and Wilgus U.S. Ser. No. 431,913, filed concurrently herewith and commonly assigned, titled RADIATION-SENSITIVE SILVER BROMOIODIDE EMULSIONS, PHOTOGRAPHIC ELEMENTS, AND PROCESSES FOR THEIR USE, which is a continuation-in-part of U.S. Ser. No. 320,909, filed Nov. 12, 1981, now abandoned, more fully discussed below, discloses high aspect ratio tabular grain silver bromiodide emulsions wherein a higher concentration of iodide is present in an annular region than in a central region of the tabular grains.
Mignot U.S. Ser. No. 320,912, filed Nov. 12, 1981 and commonly assigned, titled SILVER BROMIDE EMULSIONS OR NARROW GRAIN SIZE DISTRIBUTION AND PROCESSES FOR THEIR PREPARATION discloses high aspect ratio tabular grain silver bromide emulsions wherein the tabular grains are square or rectangular in projected area.
Maskasky U.S. Ser. No. 431,455, filed concurrently herewith and commonly assigned, titled SILVER CHLORIDE EMULSIONS OF MODIFIED CRYSTAL HABIT AND PROCESSES FOR THEIR PREPARATION, which is a continuation-in-part of U.S. Ser. No. 320,898, now abandoned, filed Nov. 12, 1981, discloses a process of preparing tabular grains having opposed major crystal faces lying in {111} crystal planes and, in one preferred form, at least one peripheral edge lying perpendicular to a &lt;211&gt; crystallographic vector in the plane of one of the major surfaces. Thus, the crystal edges obtained are crystallographically offset 30.degree. C. as compared to those of Wey. Maskasky requires that the novel tabular grains be predominantly (that is, at least 50 mole percent) chloride.
Wey and Wilgus U.S. Ser. No. 431,854, filed concurrently herewith and commonly assigned, titled NOVEL SILVER CHLOROBROMIDE EMULSIONS AND PROCESSES FOR THEIR PREPARATION which is a continuation-in-part of U.S. Ser. No. 320,899, filed Nov. 12, 1981, now abandoned, discloses tabular grain silver chlorobromide emulsions in which the molar ratio of chloride to bromide ranges up to 2:2.
Maskasky U.S. Ser. No. 431,855, filed concurrently herewith and commonly assigned, titled CONTROLLED SITE EPITAXIAL SENSITIZATION, which is a continuation-in-part of U.S. Ser. No. 320,920, now abandoned, filed Nov. 12, 1981, discloses high aspect ratio tabular grain emulsions wherein silver salt is epitaxially located on and substantially confined to selected surface sites of the tabular silver halide grains.